In this article I address the implications of Thomas Piketty's book Capital in the Twenty-First Century for our understanding of inequality and sustainability, drawing upon Amartya Sen's capability approach, and the revival of classical political economy it brings. I argue that Piketty's contribution is a significant one which has the potential to lead economic analysis in a more fruitful direction. But its potential becomes much greater if its empirical analysis is combined with the revival of classical political economy undertaken by Sen and other authors, rather than with marginalist theory.